An apparatus for testing the tack time of an epoxy prepreg glass cloth strip is provided which enables the simple and automatic measurement of the tack time of a prepreg sample. The apparatus enables the tack time test to be uniformly repeated with various samples and provides controlled pressures and constant timing sequences.
The tack time test is a quantitative measurement which relates to the degree of cure of epoxy impregnated fiberglass cloth. Epoxy prepregs such as bonding sheets and multilayer laminates are utilized for example, in the fabrication of printed circuit boards. In order to provide a standard reference for comparing the amount of partial curing of various prepregs, the United States Military developed a "tack time test" procedure which is used by manufacturers to classify their product. The tack time test as recited in Mil G-55636A at 4.6.8, requires that a strip of epoxy prepreg of approximately one inch in width and ten inches in length be wrapped around one end of a tongue depressor adjacent its distal end. A second tongue depressor is employed to secure the free end of the prepreg by positioning it directly over the first tongue depressor and fastening the two tongue depressors together at their middle. The prepreg is then placed firmly against a steel platen which is heated to 171.degree..+-.1.5.degree. C., while simultaneously initiating a timing sequence. At ten second intervals, the specimen of the prepreg is lifted from the heated platen for one second and the resin is observed to determine the presence of tackiness. As the epoxy resin approaches the gel point, it becomes increasingly tacky until it begins to string. The "tack time" is a measurement of the length of interval starting from the initiation of the test until distinct and continuous stringing occurs between the prepreg and the heated platen when the prepreg specimen is lifted off the platen.
The methods and assemblies for conducting the above described tack time test, which was established to provide uniformity throughout the field, suffered from certain shortcomings. More specifically, makeshift attempts to repeatedly carry out the above described test often failed to achieve the uniformity necessary to enable purchasers to rely on the results. Stated differently, absent an assembly to carry out the tack time test, many variables are often introduced into the procedure which affect the consistency of the results. For example, the tack time of a particular specimen will vary if the amount of pressure which is applied to the prepreg while it is against the heated platen is varied. In addition, if there is deviation in the distance to which the prepreg is withdrawn from the platen, or in the amount of time the prepreg is separated from the heated platen, the point when stringing will first be observed will differ. Further, as can be appreciated, it is relatively difficult for an operator to move the prepreg into and out of contact with the platen and observe the prepreg for tackiness while simultaneously keeping track of specific time intervals and total elapsed time. Therefore, it would be desirable to provide an assembly which simplifies the testing of tack time and which can be used in aiding the standardization of the industry.
Accordingly, it is an object of the subject invention to provide a new and improved assembly for testing the tack time of epoxy prepreg glass cloth strips which sequentially moves a prepreg specimen into and out of contact with a heated platen while simultaneously keeping a record of elapsed time until continuous stringing is observed.
It is a further object of the subject invention to provide a new and improved assembly for testing the tack time of epoxy prepreg which can consistently operate with fixed time intervals and produce uniform pressures to enable the accurate reproduction of test data.
It is another object of the subject invention to provide an assembly for testing the tack time of epoxy prepreg wherein the distance which the prepreg is lifted away from the platen, during testing, is accurately controlled.